Electric or hybrid machines are widely used as alternatives to their mechanical counterparts in industrial applications due to their higher efficiency and lower maintenance requirements. A high voltage power system is required for driving an electric machine that incorporates a high voltage (HV) electrical drive motor and electric auxiliaries. High voltage, used herein and which may also be referred to as hazardous voltage, is a practical voltage potential used for electric drive machines. This generally includes ac and dc voltages greater than 50 volts. The integrity of the high voltage power system is critical to the reliability of the machine. Typically, the machine frame will be electrically isolated from the terminals or conductors of the high voltage electrical components in the high voltage power system.
Under normal conditions, leakage currents on the order of microamps exist between the conductors of the high voltage electrical components and the machine frame, and accordingly, the leakage resistances between the conductors of the high voltage electrical components and the frame are normally very high. Under such conditions, no ground fault exists in the high voltage power system. However, electric current from a high voltage electrical component may leak to a machine frame and cause a ground fault. Such leakage currents, when significant, may be an indication of machine component fatigue or failure of a conductor's insulation. In order to ensure the proper operating conditions of the machine, it is necessary to detect ground faults such as an electrical leakage between the conductors of the high voltage electrical components and the machine frame.
At least one system and method has been implemented to detect ground fault. For example, U.S. Pat. No. 7,292,042 to Morita et al. (“the '042 patent”) discloses a ground fault detector and detection method for a vehicle. The ground fault detector described in the '042 patent is configured to detect ground faults on a plurality of electrical equipment units that are connected to the high-voltage battery. In the ground fault detector, one side of a coupling capacitor is connected to the plus terminal of a DC power source, and the other side of the coupling capacitor is connected to a central controller that applies a square-wave signal to the capacitor and detects a corresponding voltage. The ground fault detector of the '042 patent may determine the cause of the occurrence of a ground fault after detecting the presence of the ground fault.
Although the ground fault detector described in the '042 patent may be effective to some extent for detecting ground fault, it does not address detecting ground faults associated with isolated electrical components in the high voltage power system because these components may not be connected to a system-level ground fault detector. For example, an electric or hybrid electric machine may include high voltage motor controllers, power converters, or a distributed high voltage electrical system. These devices or systems may include galvanically isolated components such as, for example, power supplies or transformers. Therefore, a system-level ground fault detection system, such as the one described in the '042 patent may be incapable of detecting ground faults associated with the isolated components and/or isolated parts of a wiring system.
Furthermore, the ground fault detector described in the '042 patent may not function properly when the data communication between the electrical equipment units and the central controller fails. For example, the wiring systems and electrical components may be part of a machine control network where ground fault information is communicated to alert an operator or other systems. Therefore, in the event of a ground fault and a data communication failure (also known as a double failure condition), the operator of the ground fault detector described in the '042 patent may not be notified of the fault. As a result, damage to the high voltage power system may occur and the machine may be rendered inoperative.
The disclosed system and method for testing power transistors is directed towards overcoming one or more of the shortcomings set forth above.